FIG. 8 is a cross-sectional view showing a prior art semiconductor laser having a thyristor current confinement structure. In FIG. 8, reference numeral 1 designates an n type InP substrate. An n type InP cladding layer 2, an undoped InGaAsP active layer 3, and a first p type InP cladding layer 4 are successively disposed on the n type InP substrate 1. A ridge structure 15 includes a part of the substrate 1, the n type cladding layer 2, the active layer 3, and the first p type cladding layer 4. A p type InP current blocking layer 5 and an n type InP current blocking layer 6 are successively disposed on the substrate 1 at opposite sides of the ridge structure 15 so as to bury the ridge structure 15. A second p type InP cladding layer 7 is disposed on the n type InP current blocking layer 6 and the first p type cladding layer 4. A p type InGaAsP contact layer 8 is disposed on the second p type InP cladding layer 7. An n side electrode 9 is disposed on the rear surface of the substrate 1 and a p side electrode 10 is disposed on the p type InGaAsP contact layer 8. The second p type InP cladding layer 7, the n type InP current blocking layer 6, the p type InP current blocking layer 5, and the n type InP substrate 1 constitute a current confinement structure having a p-n-p-n structure, i.e., a thyristor structure.
This prior art semiconductor laser device operates as follows.
When a plus voltage and a minus voltage are applied to the p side electrode 10 and the n side electrode 9, respectively, i.e., a forward bias voltage is applied across the electrodes 10 and 9, a current flows from the p side electrode 10 through the contact layer 8 and the second p type cladding layer 7 toward the substrate 1 and is injected into the active layer 3, whereby laser oscillation occurs in the active layer 3. Since a p-n junction is formed on opposite sides of the active layer 3 by the current blocking layers 5 and 6, a reverse bias voltage is applied to this p-n junction during operation of the laser, so that the current is concentrated in the active layer 3. Since this laser device has a thyristor current confinement structure comprising the current blocking layers 5 and 6, the n type InP substrate 1 and the second p type InP cladding layer 7, leakage current flowing outside the active layer 3 is further reduced and more current is concentrated in the active layer 3.
However, even in the semiconductor laser device which has the thyristor current confinement structure at both sides of the active layer as shown in FIG. 8, the thyristor structure is sometimes turned on due to rising in ambient temperature, crystalline defects, rising temperature in the vicinity of the active layer during high power output operation, or the like. Further, an increase in the driving current sometimes causes a reduction in the current blocking effect, and it may increase the leakage current and cause a reduction in the laser output power, a reduction in the linearity of the light output vs. current characteristic or the like. Japanese Published Patent Application No. 61-214591 discloses a semiconductor laser device which further improves the current blocking effect. In this laser device, a base electrode is formed on a base region of a p-n-p transistor constituted by a p type cladding layer, an n type current blocking layer and a p type current blocking layer in a thyristor current confinement structure, and a reverse bias voltage is applied between emitter and base to reduce the injection current in the p-n-p transistor. In this structure, however, an external circuit is required for applying the reverse bias voltage between the emitter and base, resulting in a complicated laser driving circuit and a large-sized device.